Gender differences in cardiovascular function are well known. Hormones and related receptors are critically important factors, but clinical studies are revealing potential gender differences in afferent mediated autonomic nervous system (ANS) function. Some ANS assessments of cardiovagal reflexes have proven to be sexually dimorphic. In this study we investigate hypotheses related to possible neuroanatomical, neuophysiological and biophysical differences between aortic baroreceptors (BR) in male and female rats. A gender-related bias in aortic BR fiber type, pressure encoding and neurohormonal regulation may reveal as yet unrecognized mechanisms associated with noted gender differences in integrated cardiovagal control. This proposal builds upon our previous studies quantifying the differential composition of ionic channels in myelinated and unmyelinated BR afferents and the manner in which this defines the strikingly different reflex control of heart rate and blood pressure evoked by these neuroanatomically distinct afferent pathways. The first aim examines gender-related differences in rat aortic BR afferents. Morphometric analysis of aortic BR fibers and study of fluorescently identified aortic BR neurons (ABN) gives preliminary evidence that female rats have ~50% more myelinated BR, revealing, for the first time, a functionally distinct subtype of low threshold myelinated ABN rarely present (~2%) in age-matched males. The second aim quantifies the neuromodulatory capacity of estradiol (E2) upon this unique subtype of BR afferents. Nerve recordings show that E2 can increase the pressure-dependent discharge of single myelinated aortic BR fibers from female rats. Physiological levels of E2 (0.1 - 1 nM) acting, at least in part, via membrane bound estrogen receptors can selectively increase the excitability of this ABN subtype in female rats. Interestingly, E2 had no effect upon unmyelinated ABN from either gender. The third aim determines if E2 can alter K+ ion channel function in a manner consistent with the observed increased excitability. Our preliminary data show that, unlike all myelinated ABN in males and the balance of myelinated ABN from females, this unique subset expresses BKCa channels that provide ~25% of the whole cell potassium current. We further show that E2 selectively inhibits this BKCa current, offering one potential mechanism for the E2 sensitization of myelinated BR excitability in female rats. Finally, in aim four we determine if E2 can alter monosynaptic transmission of BR afferents onto 2nd-order BR neurons in the NTS. In male rats, monosynaptic transmission of myelinated afferents to the NTS does not involve BKCa channels. In stark contrast, E2 in female rats can increase monosynaptic transmission of myelinated afferent pathways with companion studies implicating a role for BKCa channels. Gender-related differences in the neural integration of BR sensory information from the afferent terminal through to 2nd-order BR neurons in the NTS could potentially lead to novel advances in the management of cardiovascular health and disease in the female population.